1. Field of the Invention
The present invention relates to wireless communications and, in particular, to a technique for determining uplink power used for transmissions between a base station of a system such as the general packet radio service (GPRS) system and one or more mobile stations of the system.
2. Description of Related Art
A variety of wireless communications systems are increasingly being employed. In cellular systems, for example, a network of base transceiver stations (BTS) is used to provide wireless links with mobile stations or units (e.g., cell phones). The mobile stations, sometimes referred to simply as mobiles, typically communicate via either analog or digital modulated radio frequency (RF) signals with the base station, which is itself connected to an external telephone or data network.
A variety of digital cellular networks and telecommunications standards are in use, such as GSM (Global System for Mobile Communication), and GSM derivatives (e.g., DCS 1800, PCS 1900, etc.). GPRS is an emerging standard that adds general Internet Protocol (IP) data communication, such as high-speed Internet and email data services, to GSM networks. GPRS uses a packet-mode technique to transfer high-speed and low-speed data and signaling in an efficient manner over GSM radio networks. The packet radio principle of GPRS can be used for carrying end user's packet data protocol (such as IP and X.25) information from/to a GPRS terminals to/from other GPRS terminals and/or external packet data networks. GPRS is defined by various ETSI (European Telecommunications Standards Institute) specifications such as ETSI GSM 05.08 version 6.5.0 Release 1997, “Digital cellular telecommunications; system (Phase 2+); Radio subsystem link control”; and ETSI GSM 04.60 version 6.4.0 Release 1997, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol.” See <http://www.etsi.org>.
GPRS uses a time division multiple access (TDMA) scheme. In a TDMA scheme, over a given RF channel, each mobile station in a cell transmits and receives (to and from the base station) audio data and non-audio data packets during dedicated time slices or time slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiple access (CDMA), and combinations of such schemes. In GPRS, the allocation of GPRS radio channels is flexible: from 1 to 8 radio interface timeslots can be allocated per TDMA frame. Timeslots are shared by the active users, and uplink and downlink timeslots are allocated separately. The downlink refers to transmissions from the BTS to one or more mobile stations, while uplink refers to transmissions received by the BTS. The radio interface resources can be shared dynamically between speech and data services as a function of service load and operator preference. Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150 kbit/s per user.
In GPRS systems, timeslots are further subdivided into blocks. For example, one block of data is transmitted by a base station on a timeslot every 20 ms. These data blocks, sometimes referred to as RLC/MAC blocks, contain a number of bits of data (e.g., 456 physical layer bits). A block can be intended for a particular mobile station. In addition, each block contains a header containing control information called the uplink state flag that must be decoded by all mobiles in the cell of the BTS which are sending data uplink.
To exploit the wide range of carrier-to-interference (C/1) ratios available at different locations within a cell, GPRS networks employ four different airlink coding schemes. GPRS's “lowest rate” code, CS-1, employs a relatively high number of redundancy bits and offers a maximum LLC-layer throughput of 8 kbps/timeslot. The high level of redundancy present in blocks encoded using CS-1 ensures that mobile stations at the fringes of a cell, where C/I levels are typically lowest, are able to send and receive data. In contrast, the “highest rate” code, CS-4, offers maximum LLC-layer throughputs of 20 kbps/timeslot. Because of the small number of redundancy bits added to each block encoded with CS-4, however, airlink errors can be detected, but not corrected. As a result, CS-4 offers the best airlink performance at relatively high C/I ratios. The remaining two GPRS coding schemes offer maximum LLC-layer throughputs of 12 kbps/timeslot (CS-2) and 14.4 kbps/timeslot (CS-3).
By monitoring the quality of the airlink, mobile stations and the GPRS network can select the coding scheme that offers the best performance. The process of dynamically selecting the coding scheme based on airlink quality is called link adaptation. Quality can be measured, for example, by measuring the bit error rate (BER) or block error rate (BLER) of the channel.
Dynamic power control is an important tool for mitigating co-channel interference in wireless networks. In GPRS networks, keeping co-channel interference levels low holds the promise that high rate coding schemes can be used over the airlink. The lower interference levels achieved by power control can result in higher airlink throughputs over larger portions of the cell, potentially increasing a cell's data traffic carrying capacity. Effective GPRS power control also ensures that timeslots used for GPRS do not cause unacceptable levels of interference to timeslots used for voice calls in co-channel neighbor cells.
Circuit-switched (e.g., voice) GSM and GPRS use airlink resources in dramatically different ways. Circuit-switched GSM mobile stations have dedicated use of a single timeslot (or half a timeslot, for halfrate users) for the entire duration of a call-periods of time that typically last tens of seconds to minutes. Additionally, these circuit-switched GSM mobile stations report channel quality measurements roughly twice a second.
In contrast, GPRS users share a single timeslot simultaneously with several users in a cell. A single user may transmit and receive GPRS data over multiple timeslots simultaneously. GPRS radio link connections, known as temporary block flows (TBFs), can be short-lived, lasting less than a few hundred milliseconds. GPRS mobile stations report downlink airlink quality measurements in RLC/MAC acknowledgement messages, typically in response to being polled by the base station setting a poll bit in a downlink block. Since the GPRS network is free to poll for these acknowledgement messages at will, airlink quality measurement reports sent by GPRS users will tend to be more sporadic than in circuit-switched GSM networks.
Power control “mistakes” in circuit-switched GSM and GPRS have different consequences. Inadequate-power control for circuit-switched calls can lead to dropped calls, service disruptions which are extremely annoying for users and network operators alike. Inadequate power control for GPRS mobile-stations can cause high BLERs (block error rates), or, at worst, broken TBFs. Power control errors in GPRS networks increase packet delays and decrease user throughputs, thus causing service degradation rather than wholesale service disruption.
GPRS's uplink power control mechanisms allow the network to tune the uplink transmit power used by each mobile station transmitting uplink RLC/MAC blocks. Uplink power control provides an important added benefit: transmit power used by each GPRS mobile station can be reduced to levels adequate to achieve proper airlink performance, and no higher. Transmit power can be kept as low as possible without sacrificing airlink throughput, giving users peak airlink performance without unnecessarily draining the mobile station's battery.